WHAT MAKES IT ROTATE?
AS THE GREAT SEPTEMBER GALE of 1815 threw boats down the streets of Providence, Rhode Island, John Farrar watched the winds in Boston swing around the compass from northeast to east to south in a matter of a few hours. Records later showed that around the same time in New York, the wind was shifting, too, but in different directions. Farrar deduced that the whole storm must have been rotating, like a giant turntable.
Even now, 200 years later, this phenomenon (what meteorologists call a hurricane’s primary circulation) still looks like magic, because there’s absolutely no force acting to spin the storm. And it is, in a sense, an illusion.
Because Earth is a sphere, the “ground” at the equator, where the planet is fatter, has farther to rotate in the same amount of time than the ground at, say, 40 degrees latitude. The wind bound for the low-pressure center of a tropical storm can travel a long way, as far as 1,000 kilometers (620 miles). Over such a long distance, the rotation of Earth appears to deflect those winds from a straight line into a curve. Scientists and mariners had seen this effect in the trade winds for centuries, but it wasn’t until the early 18th century that a meteorologist, George Hadley, worked out why: The winds don’t just have forward momentum from the atmospheric forces acting on them, they also have lateral momentum in the direction Earth is rotating.
So winds keep curving—to the right in the northern hemisphere and to the left in the southern, until they’re basically traveling in a circle. It’s called the Coriolis effect, after the French mathematician who later described it with an equation. In the case of tropical cyclones, the circling winds are also still traveling inward, so the whole system now looks like a spiral. Because of the curvature of the spiral, a parcel of air moving toward the center takes a longer route, so it travels over more warm seawater and picks up more moisture and heat. This gives the storm more power, making it essentially selfsustaining. When surface winds reach 39 mph, meteorologists start calling the cyclone a tropical storm and give it a first name. If the wind speed increases to 74 mph, according to admittedly arbitrary guidelines, it’s a hurricane.